The second round vote pitted challenger Julius Maada Bio of the opposition Sierra Leone People's Party (SLPP), against the ruling All Peoples' Congress (APC) candidate Samura Kamara, a close ally of outgoing President Ernest Bai Koroma.
Bio came out narrowly ahead in the first round of voting last month, but the result of Saturday's run-off election is not expected before Monday or Tuesday, after all the votes from 11,000 polling stations are counted in the poor West African nation.
"We are on the view that the election took place in an environment that (was) peaceful, that the process was transparent and credible," Amos Sawyer, head of the monitoring group sent by the 15-nation ECOWAS, said Sunday.
The observation mission "is satisfied with the conduct" of Saturday's vote, though there were some "issues", he added.
The West African bloc observers noted that some polling stations opened late and also voiced concern at the "sometimes intimidating" deployment of armed security agents at voting centres which "caused panic among voters".
The government had said that the tight security was to ensure a peaceful poll but the opposition feared it could dissuade some supporters from turning out.
Civil war memories run deep
According to analysts, the election results will be tight.
Bio, 53, said Saturday that the voting appeared peaceful and added he found the process to be "fair, transparent and credible".
He was credited with 43.3 percent to 42.7 for Kamara -- a margin of just 15,000 votes -- after the March 7 first round in the West African country where political loyalties are often divided along ethnic lines and traumatic memories of the 1991-2002 civil war run deep.
A total of 3.1 million people were registered to vote in the first presidential poll since a 2014-2016 Ebola outbreak which killed 4,000 people.
Also marring outgoing Koroma's decade-long tenure was a mudslide that struck the capital Freetown last year, killing hundreds of people.
International observers -- from the EU and the African Union as well as ECOWAS -- were deployed throughout the country and were also monitoring the vote counting.
One of the world's poorest nations despite huge metals and diamond deposits, Sierra Leone is recovering only gradually from war and disease. Its economy remains in a fragile state with corruption widespread in the former British colony.
An economist by training, Kamara, 66, was foreign minister until last year when he stepped down to pursue the presidency under the APC flag, despite lacking strong party support.
He has promised to deliver improvements in health, education and infrastructure.
The key to victory lies with whoever wins Kono, a diamond-rich district in the east of the country generally regarded as a "swing state," Abu said.
The runoff had been scheduled for last Tuesday but was delayed so the High Court could hear an APC complaint of ballot tampering.
The court lifted an injunction on Monday, but the National Electoral Commission said it needed four extra days for preparation.
Bio has blasted the government's closeness to China, while Kamara is presenting himself as a continuity candidate.
The campaign was characterised by ugly verbal exchanges and sporadic violence with Bio accusing the APC of using police intimidation against his party.
Police have reported a string of attacks on candidates and supporters on both sides since the first round after which Kamara declared that "the safety and security of Sierra Leone is in our hands."
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